This application relates generally to the fields of diffraction control, high contrast imaging, and coronagraphy. More specifically, this application discloses an occulter for creation of very deep shadows across optimally small angles.
Sometimes faint objects of interest appear close to much brighter sources of less interest. All optics contain some level of scattering and diffraction that can swamp the faint signal with stray light from the bright source. As the angular separation becomes smaller and the ratio of brightness becomes larger, seeing the faint object gets rapidly more difficult.
High contrast imaging has a number of applications including capturing an image when the target is actually trying to blind the observer, and attempting to image reflections close to a bright source. Particular applications considered herein address revealing planets circling other stars. To find and directly observe Earth-like planets, one needs to study stars as distant as 10 parsecs. The Earth is 1010 times fainter than the Sun and at 10 pc is less than a tenth of an arcsecond away.
An occulter is an opaque or partially transmitting mask that is placed in the field of view of a camera or telescope. It throws a shadow of the bright source onto the optic. Occulters have the singular property that they remove most of the radiation from the bright source before it enters the imaging optics. Light from the faint source enters unimpeded and may be viewed without interference from the bright one.
The problem of diffraction in small angle shadows is very old. In 1818 the presence of Arago's Spot (a concentration of diffracted light on the axis of a small, round occulter) convinced the scientific world of the validity of the Fresnel-Huygens wave formulation of light. But often the light diffracting around the edge of an occulter lowers contrast and ruins the performance of the system.
Occulters and coronagraphs have been in operation for many years, and the use of spokes (like the teeth on a gear) have been long known to improve the diffraction properties of an occulter, such as described in Purcell, J. D., Koomen, M. J., Coronagraph with Improved Scattered-Light Properties”, Report of NRL Progress, US GPO, Washington, D.C. (1962), the entire disclosure of which is incorporated herein by reference for all purposes. These designs typically achieved suppression ratios of 105 or higher.
In Spitzer, L., “The Beginnings and Future of Space Astronomy”, American Scientist, 50, 473-484 (1962), the entire disclosure of which is incorporated herein by reference for all purposes, the possibility was suggested of using an occulter in space in concert with a large space telescope to suppress starlight and improve contrast in telescopes near to the parent star. In Marchal, C., “Concept of a space telescope able to see the planets and even the satellites around the nearest stars”, Acta Astronautica, 12, 195-201 (1985), the entire disclosure of which is incorporated herein by reference for all purposes, a more thorough analysis was presented with the goal of revealing exoplanets. He discussed flower shaped occulters, but his apodization function could only achieve 10−5 across a tenth of an arcsecond and required a full arcsecond to achieve 10−10. Since we need 10−10 across a tenth of an arcsecond to fully reveal exoplanets, his suggestion was never implemented.
Since the discovery that planets abound around the nearby stars as described in Seager, S., “The Search for extra-solar Earth-like planets,” Earth and Planetary Science Letters, 208, 113-124 (2003), the entire disclosure of which is incorporated herein by reference for all purposes, interest in direct observation of exoplanets has grown. NASA has been funding studies of the Terrestrial Planet Finder, a telescope of such consummate perfection that it can focus light to 10−10 contrast across a tenth of an arcsecond. This telescope features a monolithic mirror 4×8 m in extent and an internal coronagraph. Studies are showing that cost is very high and it will be difficult to achieve low scatter over a broad spectral band.
A more recent look at occulters for the planet-finding application showed some promise as described in Copi, C. J., and Starkman, G. D., The Big Occulting Steerable Satellite (BOSS), Astrophysical Journal, 532, 581-592 (2000), the entire disclosure of which is incorporated herein by reference for all purposes. They used a transmitting starshade and a more generalized apodization function. They claim starlight suppression as good as 4×10−5 with potential for even higher. However, the scatter caused by the transmitting shade creates a practical limit near 10−5 anyway.
The direct observation of planets has been impeded by a general need in the art for an occulter that meets all of the conditions for practicality in terms of suppression ratio, cost, binary, size, and tolerance.